[Java][Client][Vertx] Use Vertx reference instead of static class in ApiImpl template

[ricardozanini]

Fix #8492

@bbdouglas (2017/07) @sreeshas (2017/08) @jfiala (2017/08) @lukoyanov (2017/09) @cbornet (2017/09) @jeff9finger (2018/01) @karismann (2019/03) @Zomzog (2019/04) @lwlee2608 (2019/10) @nmuesch (2021/01)

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[wing328]

LGTM after reading more about vertx.getOrCreateContext vs Vertx.currentContext